1. Field of the Invention
This invention relates to systems for driving an information carrier disk in rotation. The invention is more particularly concerned with drive systems in which the speed of rotation of a system is governed by a control loop comprising an optical detector for detecting the movement of rotation of the disk spindle. Said spindle can also be driven by a direct-current motor.
2. Description of the Prior Art
Rotation of the spindle is detected optically by mounting a wheel on the latter, the wheel being provided with a set of teeth having a uniform pitch; this set of teeth is placed on the path of a light beam and a photodetector device collects the beam modulated by the teeth. The inherent drawback which is attached to the use of a coding wheel arises from defective mounting of this latter on the spindle. These defects result in eccentric displacement of the wheel with respect to the spindle, to which there can be added another eccentric displacement caused by imperfect centering of the information carrier disk at the opposite end of said spindle. During the process involved in obtaining a precut disk, it is known that the axis of the tracks and the mechanical axis of the disk do not coincide. A difference of the order of 100 microns between the two axes is commonly observed and this results in an appreciable variation in linear velocity during rotation of the disk. When the control loop adjusts the angular speed of rotation of the spindle, a track carried by the information disk is liable to be scanned at a variable rate. This explains the difficulty involved in transcribing an isochronous signal on said track.